SURPRISE, AZ (3TV/CBS 5) – A newly released report is shedding light on an explosion at an APS storage unit in Surprise that sent four Peoria firefighters to the hospital with broken bones, chemical burns and cuts.

It happened the night of Friday, April 18 – nearly four months ago -- near Deer Valley Road and Grand Avenue.

Video provided by APS this week showed workers removing batteries from the energy storage unit, which is part of the 'utility's clean-energy project.' It's designed to store unused power generated by homes with solar panels so the solar energy can be used after the sun goes down.

"We're basically on our way to discharge all of the energy that's left in the energy storage unit," APS spokesman Scott Bordenkircher said.

While APS is still investigating the explosion, Arizona's Family obtain a report by the Surprise Fire Department.

According to that report, firefighters on a hazardous-materials crew from Peoria approached the building armed with gas detectors and a hose line. The team – comprised of Peoria Firefighter Jake Ciulla, Fire Engineer Justin Lopez, Firefighter Matt Cottini and Cpt. Hunter Clare – said that temperatures and gas meter readings were lower than when they first arrived. They decided to open the door of the building to get a look inside.

Moments later, something exploded and the mayday call went out.

"I do know it just knocked their helmets and face masks off of their bodies... that's how the one firefighter, the facemask just crushed into his face and off of his face. The helmets are laying (sic) on the ground with all the interior portion of the helmet gone. They're just shells on the ground," Mike Selmer with the Peoria Fire-Medical Department said the next day.

Two of the men were flown to the hospital. The other two were taken by ground ambulance.